Ethylene polymerized with acrylic acid to form copolymers is taught, for example, in U.S. Pat. No. 2,391,218; U.S. Pat. No. 3,239,370; U.S. Pat. No. 3,520,861 and U.S. Pat. No. 4,351,931.
Ethylene polymerized with higher olefins, by the action of a coordination catalyst, to form linear low density ethylene polymers (a.k.a. LLDPE), is taught, for example, in U.S. Pat. No. 4,076,698; U.S. Pat. No. 3,058,963; U.S. Pat. No. 2,846,425; U.S. Pat. No. 2,905,645; and U.S. Pat. No. 2,396,785.
Blends of certain olefin polymers and ethylene/acrylic acid copolymers are taught, for example, in U.S. Pat. No. 3,410,928 and U.S. Pat. No. 3,600,468.
There are, basically, two types of olefin polymerization techniques for preparing high molecular weight olefin polymers and copolymers. The oldest commercial technique involves high pressure, high temperature, and the use of a free radical initiator, such as a peroxide; these type polymers are generally known as low density polyethylene (LDPE) and are also known as ICI-type polyethylenes. These LDPE polymers contain branched chains of polymerized monomer units pendant from the main polymer "backbone" and generally have densities in the range of about 0.910-0.935 gms./cc.
The other commercially-used technique involves coordination catalysts of the "Ziegler" type or "Phillips" type and includes variations of the Ziegler type, such as the Natta type. The catalysts may be used at very high pressures, but are generally used at very low or intermediate pressures. The products made by these coordination catalysts are generally known as "linear" polymers because of the substantial absence of branched chains of polymerized monomer units pendant from the main polymer "backbone," and they are also generally known as high density polyethylene (HDPE). It is these "linear" polymers to which the present invention pertains. Linear polyethylene (HDPE) ordinarily has a density in the range of 0.94 to 0.98 gms./cc.
In particular, the present invention pertains to "linear" type ethylene polymers wherein ethylene has been polymerized along with minor amounts of alpha, beta-ethylenically unsaturated alkenes having from 3 to 12 carbons per alkene molecule, preferably 4 to 8. The amount of the alkene comonomer is generally sufficient to cause the density of the polymer to be substantially in the sane density range as LDPE, due to the alkyl side chains on the polymer molecule, yet the polymer remains in the "linear" classification; they are conveniently referred to as "linear low density polyethylene" (LLDPE). These polymers retain much of the strength, crystallinity, and toughness normally found in HDPE homopolymers of ethylene, but the higher alkene comomers impart high "block" characteristics to extrusion-cast films and the high "slip" (i.e. low coefficient of friction) characteristic inherently found in HDPE is diminished.